Sometimes the motivations of user are not directly related to behaviors that the designer is attempting to encourage.
For example, if Twitter discovered that following 30 people is more likely to lead to long term engagement, then they will try to encourage that regardless of the users actual motivation. In the long run, this could benefit the user too, but the user doesn't know this and usually would not take action based off a purely rational argument. So the designer needs to encourage this behavior another way, perhaps by indirectly harnessing other user motivations.
We have terms like "reduce friction" that encompass some of the things that a UX designer does. Is there an analogous term for this sort of design challenge?